1. Introduction

Nanosecond-duration lasers are in most cases attained by Q-switching techniques. By using high-power or high-energy fiber amplifiers, nanosecond (ns) Q-switched lasers were amplified above 100 W average power [1,2] or several millijoules per pulse [3]. Recently, high-power fiber amplifiers have been spectrally combined up to an average power of 1.1 kW [4]. Synchronous pulse amplifiers could be accumulated in the time domain to attain even higher powers. However, Q-switched lasers are typically synchronized by electronic triggers with insurmountable timing jitters limited by the electronic circuits. Precise temporal control techniques well-developed for ultrashort lasers are unfortunately inapplicable to ns Q-switched lasers. In principle, accurate synchronization of ns laser pulses can be achieved by phase-locking ns lasers to the same ultrafast laser trigger [5]. High-energy ns lasers synchronous with separate picosecond or femtosecond lasers are desired in optical parametric chirped pulse amplification and many other high-energy physics experiments [6–9]. In particular, wavelength-tunable and duration-adjustable high-power ns laser sources are of great interest for material processing, remote sensing, laser-matter interactions, optical parametric amplifiers, and fast ignition of inertial confined fusion [10–14]. Recently, it has been demonstrated that flat-top ns rather than picosecond or femtosecond mode-locking could be realized by virtue of peak-power clamping effects in fiber lasers which may facilitate a precision temporal control of ns mode-locked fiber lasers [5,15,16].

In this paper, we report on the experimental realization of a self-started passively mode-locked fiber laser that generated flat-top ns pulses of narrow spectral bandwidths suitable for high-power amplification. The pulse duration could be changed from 1 to 15 ns by varying the pump power or laser polarization evolution in the cavity. The spectrally narrowband flat-top ns pulses could be tuned from 1033 to 1053 nm by changing the nonlinear polarization rotation in the fiber laser cavity, which were further amplified up to 280 W by using an ytterbium-doped single-mode fiber preamplifier and a two-stage large-mode-area Yb-doped double-clad-fiber (YDCF) power amplifier. This is to the best of our knowledge the highest average power ever reported for flat-top ns fiber laser around 1 μm. By appropriately controlling the spectral bandwidth of the ns laser seed, negligible spectral or temporal distortions were observed during the high-power amplification and as a consequence, the ns pulses remained the same temporal flat-top profiles after high-power fiber amplifications. The high-power flat-top ns laser pulses may serve as a promising candidate for pumping optical parametric chirped-pulse amplification [17], which greatly relaxes the strict synchronization requirements between the seed and pump pulses.

2. Flat-top nanosecond fiber laser

As schematically sketched in Fig. 1 , the tunable ns fiber laser used for our experiments employed a unidirectional ring cavity configuration, which was pumped by a pigtailed laser diode at 976 nm. The fiber section consisted of 103 m single-mode fiber and 1.2 m Yb-doped gain fiber with a high Yb-doping concentration to allow the pump absorption of 250 dB/m and a mode-field diameter of 3.6 μm at 1000 nm. The fiber laser could self-start mode-locking at the fundamental repetition rate of 1.9 MHz by nonlinear polarization rotation in the single-mode fiber. As the fiber laser was operated in normal-dispersion region, the output pulses were stretched in the single-mode fiber after the gain fiber [18], while the nonlinear polarization evolution was controlled by using quarter- and half-wave plates. The laser output was taken directly from the polarizing beam splitter and monitored by a fast photodetector with a rising time of less than 500 ps and a sampling oscilloscope of 6 GHz bandwidth. Such a laser configuration exhibited a cw lasing threshold of 280 mW pump power. By properly setting the polarization state of the laser cavity, self-started mode-locking could be initiated at a pump power of 330 mW. Once mode-locking was self-started, it remained stable for several hours.

 

Fig. 1 Schematic setup of the flat-top ns fiber laser and high-power Yb-doped double-clad fiber amplifier. SMF, single-mode fiber; w. p., waveplate; YDCF, Yb-doped double-clad fiber.

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In the long-fiber cavity, the round-trip transmission was determined by the power-dependent beat length. Once the pulses in the fiber reached the peak power for nonlinear polarization switching at which the round-trip transmission reached its maximum, the long-cavity fiber laser could be passively mode-locked to output flat-top ns pulses. As the nonlinear polarization switching was critically dependent upon the intracavity birefringence and thus changes of the quarter- and half-wave plates in the fiber cavity brought about observable changes of the flat-top ns mode-locking. Accordingly, nonlinear phase modulations in the SMF could be controlled to produce narrow lasing bandwidths, in contrast with flat-top ns pulses previously attained in ns mode-locked fiber lasers that exhibit quite broadband (bandwidth>30 nm) [5,19]. It is well-known that spectrally broadband pulses are quite difficult to amplify to high average powers without spectral or temporal distortions as high-power fiber amplification may induce observable spectral narrowing. In fiber laser with ns pulse duration, intracavity fiber dispersion played a negligible role on the output pulse profile. It was the gain competition and nonlinear phase modulation that determined the central wavelengths and net gain bandwidth of the fiber laser output. In our experiment, proper parameters of the pump power and the length of the single-mode fiber before the gain fiber were used to keep low nonlinear phase modulation in order to obtain ns pulses with narrowband lasing spectra. As a whole, interplay of the nonlinear polarization rotation (wavelength-dependent nonlinear beat length) and nonlinear phase modulation functioned equivalently as nonlinear filtering and polarization switching to self-start flat-top ns mode-locking with narrowband bandwidths and tunable central wavelengths.

By carefully aligning the quarter- and half-wave plates to change the nonlinear polarization evolution in the passively mode-locked ytterbium fiber laser with the pump power fixed at 500 mW, the central lasing wavelength could be tuned within the range of 1033-1053 nm. Typical output laser spectra are shown in Fig. 2(a) under different alignments of the intracavity polarization states. Interestingly, the narrowband flat-top ns pulses could be tuned around 1030 nm to match the gain of YDCF amplifiers, which could be used to develop large-mode-area high-power fiber amplification at high-repetition rates, or tuned at 1053 nm to match the maximum gain wavelength of Nd-doped phosphate glass laser amplifiers, which could sustain high-energy pulse amplification by using larger-aperture gain media. Figure 2(b) shows the dependence of the pulse width vs. the pump power at 1036 nm. When the emission wavelength was fixed at 1036 nm, the pump powers are 330, 369, 392, 412, 468 and 502 mW (in ascending order), leading to pulse durations of 1, 3, 6, 9, 12 and 15 ns, respectively. By changing the nonlinear polarization evolution the laser cavity, the tunable pulse durations ranges were also changed with the central lasing wavelength. The flat-top temporal profiles and the corresponding variation of the output pulse durations under different pump powers confirmed that the intensity-dependent cavity transmission reached its maximum as the pump power was increased above a certain threshold. Peak power clamping was verified by the flat-top ns output pulses and pulse duration stretching with broadened flat tops at high pump powers. The shot-to-shot fluctuation and long-term mode-locking stability were monitored by measuring the output pulse energy with a photodiode. As shown in Fig. 2(c) for the standard deviation (SD) of the output pulse energies, the pulses remained stable with a typical pulse energy fluctuation about 1%. The pulse train was recorded by a power spectrum analyzer, which clearly shows that the flat-top ns mode-locking was stably operated at 1.9 MHz [Fig. 2(d)] without any observable sideband power spectra [Fig. 2(e)]. We note that the pulse duration could be readily adjusted by varying the pump power. It is expected to further stretch the flat tops of the ns pulses merely by increasing the pump power. The ns pulse duration could also be tuned from 1 to 15 ns by continuously adjusting the quarter- and half-wave plates and thus changing the intracavity polarization state under a fixed pump power.

 

Fig. 2 Typical output laser spectra under different alignments of the intracavity polarization states (a). Temporal profiles of the output pulses under different pump powers (b). The standard deviations of the output pulse energy of the flat-top ns mode-locked laser (c). Flat-top ns mode-locked pulse train at the fundamental repetition rate of 1.9 MHz (d) and its radio-frequency output power spectrum with distinct suppression of sidebands (e).

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3. Amplification of flat-top nanosecond pulse

The advent of large-mode-area double-clad fibers with outstanding thermo-optical properties offers a promising approach to generate high-power laser pulses at high repetition rates. In our high-power ns pulse amplification system, a mode-locked Yb-doped fiber laser oscillator mentioned above was tuned to 1034 nm with 3 ns pulse duration as the seed source, matching well with the maximum gain bandwidth of Yb-doped double-clad fibers. The average output power was further amplified to 150 mW by a two-stage single-mode-fiber preamplifier. To achieving high-power flat-top ns pluses, two 1 m long Yb-doped large-mode-area photonic-crystal double-clad fibers with an active core diameter of 41 μm and an inner cladding diameter of 170 µm (NA = 0.55) was used as the gain medium. The fiber ends was polished at an angle of 8° to avoid parasitic lasing. The pump absorption for the Yb-doped fiber was 10 dB/m at 976 nm. Two pigtailed high-power diode-lasers at 976 nm with fiber core of 400 µm are employed as the pump source. 70% of the pump laser was coupled into the inner clad and 60% of the seed power was coupled into the fiber core. A high-power optical isolator was applied to avoid interaction between the two parts of the power amplifier. For each stage of fiber amplifiers, the input power was optimized for the gain so that amplified spontaneous emission could be suppressed efficiently [20,21]. The amplified laser output was monitored by the same fast photodetector with a rising time of less than 500 ps and sampling oscilloscope of 6 GHz bandwidth.

In our experiment, 3 ns pulse laser with 150 mW average power was amplified to 15 W by the first-stage power amplifier, and was finally boosted to 280 W by the second-stage power amplifier. The corresponding pulse energy was about 150 μJ. The corresponding slope efficiency for the second-stage power amplifier was 86%, as shown in Fig. 3 . The micro-structured fiber core ensured a single-mode operation during amplification. Due to the interaction of gain, nonlinearity and normal dispersion in the active fiber, the ns pulse experienced a spectral broadening during the process of parabolic amplification. Figure 4(a) shows the seed and amplified spectra at different output power for the second-stage power-amplifier. The amplified flat-top ns pulses were broadened to a full-width of half maximum (FWHM) of 9.7 nm at 280 W from 6.4 nm of the seed pulses. Without spectral narrowing, high-power parabolic amplification in the YDCF was accompanied by negligible temporal distortions of the flat-top ns pulses, as shown in Fig. 4(b). The standard deviation of the flat top of seed and amplified ns lasers was calculated as 0.3% and 0.5%, which fully demonstrated the use of YDCF would keep the flat-top characteristics of ns pulses.

 

Fig. 3 The slope efficiency for the second-stage power amplifier of the flat-top ns pulses.

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Fig. 4 The measured spectra relative intensity (a), absolute intensity (inserted), and temporal profiles (b) of the fiber ring laser and amplified pulses.

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4. Conclusion

In conclusion, we demonstrated the first scalable approach of efficient fiber-based laser system to generate high-average-power tunable nanosecond flat-top laser pulses around 1 μm. We achieved 280-W average power of nanosecond pulses with excellent beam quality. The achieved results were merely limited by the available pump power rather than nonlinear pulse distortions. Therefore further power scaling is possible even with the current system. Future investigations will focus on the passive synchronization of different high-power mode-locked laser pulses with ns and fs durations.